Suggestion: An easy way to prevent most game breaking 'Gambits'

[Bavre]

With France my advice would be to go all out cannon rush: cannon+shell techs with max chits plus building all cannons possible.
If you attack with infantery alone you'll not only suffer severe casualties, but also pump the german HQ full of xp. Apart from that trenches 2 will be around 5-6 turns in, making any further unsupported attack on the strong german units suicide.

With Russia it's a bit tricky and highly depends on how skilled/careful the CP player is. Options are feint (or vs very careless player even real) counterattack, snatching of 1 or 2 early undefended places, or just hunkering in a very defensible position. It's basically 100% opportunism and really just depends on what CP does.
Just running away does not work, because you`ll loose a lot of NM objectives (mdsmall and me already tried that).